Tracking down starflight in literature is an absorbing pastime. When I was writing my Centauri Dreams book, I found that I was vaguely familiar with many of the antecedents of today’s science fictional journeys, but a book called Wunderwelten, by Friedrich Wilhelm Mader, took me by surprise. A 1911 adventure novel for young readers, Wunderwelten imagines a sphere that, in the fashion of the time’s space fiction, was moved by antigravity in a multi-year journey to Alpha Centauri. Mader’s ship, called ‘Sannah,’ was a precursor to all the Centauri-bound starships to come.


What a delight to find Sannah emerge in the form of Sannah III in Stephen Baxter’s story “Star Call,” which appears in the recently published Starship Century. But Baxter’s updated ship is a far cry from the 50-meter antigravity vessel imagined by Mader. For one thing, it’s gifted with artificial intelligence:

I am called Sannah III because I am the third of four copies who were created in the NuMind Laboratory at the NASA Ames research base. I was the one who was most keen to volunteer for this duty. One of my sisters will be kept at NASA Ames as backup and mirror, which means that if anything goes wrong with me the sentience engineers will study her to help me. The other sisters will be assigned to different tasks. I want you to know that I understand that I will not come home from this mission. I chose this path freely. I believe it is a worthy cause.

Image: Science fiction novelist Stephen Baxter, who has revived Mader’s antique starship.

Sannah’s cause, and the thinking around it, are regularly reported back to Earth as it travels, all by means of the Star Call system, which allows people to buy a share in the mission and in return get once-a-decade message exchanges with the starship. A poignancy in these communiques emerges that reminds me of Greg Bear’s sentient starship in Queen of Angels as we begin to realize the mission is not going well and Sannah may not have all the facts.

Emergence of Pellet Propulsion

Interestingly, the stardrive on Sannah is not antimatter but what Baxter dubs a ‘Singer-Nordley-Crowl’ drive after Clifford Singer, who studied pellet propulsion technologies back in the late 1970s. The Nordley reference is to Gerald Nordley, whose own pellet propulsion methods revised and significantly upgraded Singer to allow for ‘smart pellets’ with course correction. Crowl, of course, is our own Adam Crowl, who has been writing and commenting on this site almost since its inception, and whose own entry in Starship Century is a comprehensive look at how researchers have envisioned starships in our time.

Adam, serious congratulations, buddy. I mean, to have a stardrive named after you…

Nordley was interested in nanotechnology and proposed that the problems of getting small particles moving at relativistic speeds to their target (where they would push against its magnetic field to drive it forward) could be handled by artificial intelligence and minute rockets. ‘Smart pellets’ wouldn’t be easy to send on their way but nanotechnology worked there as well. Says Crowl:

To power either system would require immense solar power-collection systems, which Nordley proposed to be built via self-replicating machines. Optimistically assuming a single self-replicating power-satellite that supplies one gigawatt of power that copies itself in a year, then within mere decades sufficient power would be available to propel a 1,000-ton starship to 0.86c at five-Gs, and a decade later a thousand such starships could be propelled per year.

0.86c is an interesting figure. Nordley told me in an interview years back that he thought the first human crew to reach Alpha Centauri would get there after a journey lasting about three years. That’s three years as experienced by the crew. Moving at 0.86c, he added, those aboard the starship would experience a time compression factor of two — half as much time would expire for them as would expire for the people left behind on Earth. Add in acceleration and deceleration time and you get the result, a three year passage (as perceived by those onboard) to the nearest star. It’s about the same amount of time it took Magellan to circumnavigate the Earth.


Image: Interstellar researcher Gerald Nordley, speaking at the Space Access 2010 Conference in Phoenix, Arizona.

Interactions with the Medium

Crowl’s paper runs through all the starship concepts I’ve ever encountered, among the most fascinating of which are the lesser known. Back in the 1970s, for example, as NASA studied the possibility of pushing a probe up to interstellar speeds using lasers, Philip C. Norem and Robert Forward went to work on the question of how to slow down a probe for rendezvous. One of Forward’s sail deceleration concepts was ingenious enough to merit separate treatment, and I’ll talk about it tomorrow as we discuss Jim Benford’s ideas on laser and microwave sails. But there are other ways of doing these things, and Norem and Forward found that a starship could be turned by using large charged wires to interact with the galactic magnetic field.

The key to this is the fact that a charged object moving through a magnetic field experiences a Lorentz force at right angles to its direction of motion and the magnetic field itself. If you give it enough time to work, the Forward/Norem method can actually slow a probe down and turn it so that it approaches the target star (from our perspective on Earth) from behind. At that point a laser beam from Earth could be trained on the starship’s sail to slow it for the rendezvous. The same method could be used in reverse to enable a return journey. The main problem is that the large turning circles require centuries of additional travel time to pull off the feat.

Both Forward and Norem were fascinated by the concept of ‘thrustless turning,’ written up by Norem in a 1969 paper. I’ll mention another aspect of this that may be germane here, a 2005 paper by Gregory Matloff and Les Johnson that studies how to use the interstellar medium not for turning but for generating power aboard the spacecraft. This could be done through the interactions between an electrodynamic tether and the interstellar magnetic field.

We might throw Freeman Dyson into the mix as well. Dyson studied propellantless braking after observing the magnetic interactions between the large inflatable satellites of the early 1960s and the plasma around them. A starship using these methods to decelerate would release electromagnetic energy that might be observable, thus allowing a search for extraterrestrial space vehicles. Crowl discusses what Dyson called Alfven braking in relation to magnetic sail concepts that emerged in the late 1980s. And it’s to sails, though not magnetic ones, that I want to turn tomorrow as we ponder Crowl’s many propulsion alternatives.

The Philip Norem paper is “Interstellar Travel: A Round Trip Propulsion System with Relativistic Capabilities,” AAS 69-388 (June, 1969). Robert Forward’s paper on Lorentz force turning is “Zero-Thrust Velocity Vector Control for Interstellar Probes: Lorentz Force Navigation and Circling,” AIAA Journal 2 (1964), pp. 885-889. Gregory Matloff and Les Johnson write about electrodynamic tether possibilities in “Applications of the Electrodynamic Tether to Interstellar Travel,” JBIS 58 (June, 2005), pp. 398-402. Cliff Singer’s first pellet paper is “Interstellar Propulsion Using a Pellet Stream for Momentum Transfer,” JBIS 33 (1980), pp. 107-115.